Why governments miscalculate your risk of flooding – Technology & Science

Think your home is safe from flooding? Unfortunately, you’re probably wrong. And the government’s estimates of your flood risk are likely wrong too, thanks in part to climate change and outdated flood maps that don’t take it into account, researchers say.

We’ve seen the extreme flooding in Texas following Hurricane Harvey — houses and semi-trucks half-submerged in swampy brown water, people paddling in boats or wading through rivers that were once streets.

A preliminary estimate of the flooding in Harris County in the Houston area by researchers at the University of California, Davis, suggests that more than half the flooding there happened outside any mapped flood zone.

‘What used to be a one-in-100 year event might now be more frequent.’
– Glenn McGillivray,  Institute for Catastrophic Loss Reduction

Unfortunately, it doesn’t take an exceptional hurricane like Harvey to flood areas beyond a known flood plain, as residents of Windsor and Essex County in southern Ontario found out in late August, when more than 5,000 homes suffered flood damage from heavy rain.

Why does so much flooding seem to happen outside known flood zones these days?

Partly, it’s because the flood maps used by municipal governments to calculate the risk of flooding are inaccurate.

“A lot of them are quite old,” said Glenn McGillivray, managing director of the Institute for Catastrophic Loss Reduction, an independent not-for-profit disaster research institute established by the property insurance industry and affiliated with Western University.

Decades-old flood maps

Many flood maps were made decades ago, when there was a federal cost-sharing program that used to help defray the cost, and haven’t been updated since.

Not only were methods for predicting flood risk more primitive then and based on less data than we have now, but other things may have changed since the maps were made.

A city may have paved over vegetation that used to absorb some of the water,…

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